The most common process of preparing or recovering selenium comprises converting selenium in a selenium-containing material to selenium dioxide (SeO.sub.2) by oxidizing roast or sulfatizing roast in a roasting furnace, dissolving the selenium dioxide in an aqueous solution, purifying said solution and reducing the dissolved selenium dioxide with sulfur dioxide (SO.sub.2) to obtain selenium of 99.9% purity. This process is widely employed because of the advantage that materials in any form can be treated. However, a large scale facility including a roasting furnace, apparatuses for selenium dioxide collection, treatment of waste gases, etc. is required, and the energy consumption is large and the cost for reduction of sulfur dioxide is very high.
Various wet processes for recovering selenium are proposed in Japanese Laid-Open Patent Publication No. 128595/80 and other publications. However, those process are limited as regards kinds of materials, condition in which selenium is present, applicability of used reagents, etc., and in most cases waste liquid treatment is required. Therefore, those processes are not so widely employed as the above-described process. In Japanese Patent Publication No. 35006/84, there is disclosed a wet process, in which a strong acid such as nitric acid, hydrochloric acid, etc. are used. In this process, however, equipment maintenance is not easy because the strong acid is used, and measures for treatment of NO.sub.x, which is generated during the operation, are required. Further, as hydrazine is used as a reducing reagent, this is not an economical process.
Japanese Patent Publication No. 9734/76 discloses a process for preparing tetrasubstituted urea, in which a primary or secondary amine is reacted with carbon monoxide and selenium to form a tetrasubstituted urea. The reaction proceeds, for instance, as follows: ##STR1##
The intermediate product of the reaction (I) and the product of the reactions (I') and (I'a) are soluble in the reaction system. Needless to say, R and R' in the above chemical formulas each stands for a hydrocarbyl group.
We noted that the above reactions are reversible and we inferred that this reaction could be utilized for recovery and purification of selenium, and we completed the present invention. For the purpose of this invention, only the first step reactions (I) or (I') and (I'a) are necessary.